The brain is an exquisitely complex organ. This complexity is not born out of its estimated 82 billion neurons, but instead in the trillions of specific connections these neurons make between one another during development called synapses. Yet, it is fundamentally unknown how the many different types of synapses form during development, nor is it known how synapse populations coordinate formation to create a working balance within in a neural circuit. This is a critical gap in the understanding of early brain development as numerous neurodevelopmental disorders, such as autism and epilepsy, are thought to arise from improper synapse formation resulting in circuit imbalances. Synapses are broadly defined into two groups: electrical synapses which form first and persist into adulthood; and chemical synapses which develop after electrical synapses and communicate via chemical neurotransmitters. While the biochemical makeup of electrical and chemical synapses are distinct, suggesting different processes regulate their formation, the autism- and epilepsy-associated gene Neurobeachin is required for both electrical and chemical synapse formation. The goal of this proposal is to 1) identify the molecular mechanisms used by Neurobeachin to regulate electrical and chemical synapse formation, and 2) determine if these mechanisms are similar to or distinct from one another. To realize this goal, the zebrafish Mauthner circuit will be used as it is ideal for dissection of Neurobeachin function during early development in vivo. Mauthner neurons make identifiable, stereotyped electrical and chemical synapses that are easily observed in developing embryos and accessible to cell biological, genetic, and biochemical investigation. Together, this proposal will unlock the shared mechanisms guiding electrical and chemical synapse formation and shed new light onto a molecule critical for brain development and implicated in various neurodevelopmental disorders ultimately leading to the design of rational therapeutic interventions. This proposal is designed to provide training in cutting edge CRISPR editing, dynamic live imaging, and sophisticated cell biology and biochemistry techniques within the zebrafish model system across early brain development at the University of Oregon. The University of Oregon is the birthplace of the zebrafish as a model organism and the preeminent institution to learn innovative zebrafish techniques among a highly collaborative and supportive environment of researchers. This experience will train the postdoctoral researcher in advanced research methodology and quantitative analysis, statistics, manuscript writing, and mentoring techniques necessary for successful growth into an independent academic research position.